accounted/lib/invoices/__tests__/late-payment-interest.test.ts

import { describe, it, expect } from 'vitest'
import {
  calculateLatePaymentInterest,
  getAnnualInterestRate,
  getReferensrantaAt,
} from '../late-payment-interest'

describe('getReferensrantaAt', () => {
  it('returns 0 for early 2022', () => {
    expect(getReferensrantaAt('2022-01-15')).toBe(0)
  })

  it('returns 0.005 for late 2022', () => {
    expect(getReferensrantaAt('2022-12-31')).toBe(0.005)
  })

  it('returns the boundary value exactly on switch date', () => {
    expect(getReferensrantaAt('2024-07-01')).toBe(0.0425)
  })

  it('returns the most recent rate for dates in 2026', () => {
    expect(getReferensrantaAt('2026-05-15')).toBe(0.025)
  })

  it('falls back to earliest entry for pre-2022 dates', () => {
    expect(getReferensrantaAt('2021-01-01')).toBe(0)
  })
})

describe('getAnnualInterestRate', () => {
  it('adds 8 percentage points to referensränta by default', () => {
    // 2026 referensränta = 0.025 → 0.025 + 0.08 = 0.105
    const rate = getAnnualInterestRate('2026-05-01')
    expect(rate).toBeCloseTo(0.105, 4)
  })

  it('uses the override rate when supplied', () => {
    const rate = getAnnualInterestRate('2026-05-01', 0.115)
    expect(rate).toBe(0.115)
  })

  it('respects override of 0 (interest-free reminder)', () => {
    const rate = getAnnualInterestRate('2026-05-01', 0)
    expect(rate).toBe(0)
  })

  it('ignores undefined override but respects 0', () => {
    expect(getAnnualInterestRate('2026-05-01', undefined)).toBeCloseTo(0.105, 4)
    expect(getAnnualInterestRate('2026-05-01', null)).toBeCloseTo(0.105, 4)
  })
})

describe('calculateLatePaymentInterest', () => {
  it('returns 0 when invoice is not yet overdue', () => {
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2026-06-01',
      asOfDate: '2026-05-15',
    })
    expect(result.amount).toBe(0)
    expect(result.days).toBe(0)
    expect(result.fromDate).toBe('2026-06-01')
  })

  it('returns 0 when asOfDate equals dueDate', () => {
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2026-05-15',
      asOfDate: '2026-05-15',
    })
    expect(result.amount).toBe(0)
    expect(result.days).toBe(0)
  })

  it('computes Räntelagen §6 default for 30 days on 10 000 kr (2026 rate 10.5%)', () => {
    // 2026-01-01 referensränta = 0.025 → annual rate = 0.105
    // interest = 10 000 × 0.105 × 30 / 365 ≈ 86.30
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2026-04-15',
      asOfDate: '2026-05-15',
    })
    expect(result.days).toBe(30)
    expect(result.rate).toBeCloseTo(0.105, 4)
    expect(result.amount).toBeCloseTo(86.3, 1)
  })

  it('computes with explicit override 5% on 10 000 kr for 30 days ≈ 41.10', () => {
    // interest = 10 000 × 0.05 × 30 / 365 ≈ 41.0959
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2026-04-15',
      asOfDate: '2026-05-15',
      overrideRate: 0.05,
    })
    expect(result.amount).toBeCloseTo(41.1, 1)
    expect(result.rate).toBe(0.05)
  })

  it('computes with explicit override 11.5% on 10 000 kr for 30 days ≈ 94.52', () => {
    // interest = 10 000 × 0.115 × 30 / 365 ≈ 94.5205
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2026-04-15',
      asOfDate: '2026-05-15',
      overrideRate: 0.115,
    })
    expect(result.amount).toBeCloseTo(94.52, 1)
    expect(result.rate).toBe(0.115)
  })

  it('uses the rate at the dueDate even when asOfDate is in a later rate period', () => {
    // Due in late 2025 (rate at 2025-12-15: referensränta 0.0325 → annual 0.1125),
    // checked in 2026 (rate 0.025 + 0.08 = 0.105). We should use the dueDate rate.
    const result = calculateLatePaymentInterest({
      overdueAmount: 10_000,
      dueDate: '2025-12-15',
      asOfDate: '2026-02-15',
    })
    expect(result.rate).toBeCloseTo(0.1125, 4)
  })

  it('throws on a negative overdue amount', () => {
    expect(() =>
      calculateLatePaymentInterest({
        overdueAmount: -500,
        dueDate: '2026-04-15',
        asOfDate: '2026-05-15',
      }),
    ).toThrow()
  })

  it('returns 0 amount when overdueAmount is 0 even if days > 0', () => {
    const result = calculateLatePaymentInterest({
      overdueAmount: 0,
      dueDate: '2026-04-15',
      asOfDate: '2026-05-15',
    })
    expect(result.amount).toBe(0)
    expect(result.days).toBe(0)
  })

  it('rounds to 2 decimals (no toFixed drift)', () => {
    // 12 345 × 0.115 × 17 / 365 = 66.1150... → 66.12
    const result = calculateLatePaymentInterest({
      overdueAmount: 12_345,
      dueDate: '2026-04-15',
      asOfDate: '2026-05-02',
      overrideRate: 0.115,
    })
    expect(result.amount).toBe(66.12)
  })
})